USB enclosure
A number of manufacturers offer portable USB hard drives that offer performance comparable to conventional ATA (IDE) drives. These external drives, called enclosures, are often composed of an embedded system translating device that connect to USB on one side and to conventional IDE, ATA, ATAPI, or SCSI drives on the other. You install a drive into the enclosure and then plug the enclosure into the computer; thus functioning as a regular USB mass-storage device.
One important advantage in making it possible to install and remove devices without opening the computer case, making it useful for external drives.